Field of the Invention
The present invention concerns a method to acquire magnetic resonance data of a first spin species and a magnetic resonance system, and in particular concerns the suppression of a signal of a second spin species in the magnetic resonance data.
Description of the Prior Art
Within the scope of magnetic resonance (MR) data acquisition of signals elicited from nuclear spins, a longitudinal magnetization is polarized in a basic magnetic field. The longitudinal magnetization is excited by an excitation pulse so that a transverse magnetization arises. This can be specifically manipulated, for example dephased and rephased so that an echo is produced. This echo can be detected as a signal in order to provide MR data. Signals from proton nuclear spins are often measured. A spatial resolution of the MR data can be generated by application of gradient pulses that produce spatially variable gradient fields.
Within the scope of the MR measurements (data acquisitions), it is possible to separate spectral components in the MR data, and thus to suppress individual components. The spectral components can original from different spin species. Such techniques often utilize the effect that the resonance frequency of nuclear spins depends on the molecular or chemical environment. This effect is designated as a chemical shift or frequency shift. Different spin species therefore have different resonance frequencies from which the measured spectrum of the MR data is composed. For example, the difference between two resonance frequencies of different spectral portions—i.e. the frequency shift—can be expressed in ppm (“parts per million”, i.e. 10-6 [sic]).
The frequency shift between proton nuclear spins in water (water signal) as a first spectral component and proton nuclear spins in fatty acid chains (fat signal) as a second spectral component is often considered. In such a case, a water MR image and/or a fat MR image—i.e. individual MR images of the two spectral components—can be determined using MR data.
For example, a water MR image in which the fat signal is suppressed can be of interest. This is of interest for a variety of clinical and/or medical applications, for example. For example, certain anatomical details or pathologies are shown in a particular manner given suppression of the fat signal, which can be essential to the assessment of the images by a radiologist. In MR spectroscopy, interesting signals (spectral lines) of specific metabolites (i.e. chemical bonds in which the resonance frequency of the protons is characteristically shifted) can be overlapped by the dominating fat signal, and thus cannot be interpreted, or can be interpreted only with difficulty. Moreover, in specific imaging methods, fat signals lead to artifacts that hinder the diagnosis. This applies in particular to echoplanar imaging, in which the fat tissue is often shown shifted by several pixels due to the frequency shift of the fat signal and the small bandwidth along the phase coding direction.
Various techniques for suppression of the fat signal, of the signal originating from the second spin species, in general are known that are based on the frequency shift. One example is the Dixon technique; see W. T. Dixon, “Simple proton spectroscopic imaging” in Radiology 153 (1984) 189-194. An additional technique is the slice selective gradient reversal technique (SSGR); see for example H. W. Park et al., “Gradient Reversal Technique an Application to Chemical-Shift-Related NMR Imaging” in Magn. Reson. Med. 4 (1987) 526-536. In the SSGR technique, use is made of the fact that, in the case of successive radio-frequency (RF) pulses that are respectively accompanied by slice selection gradient pulses with different polarity, spatial domain profiles of a flip angle of the RF pulses do not overlap (or overlap only in part) along a slice selection direction for the spin species to be suppressed. Corresponding techniques are also known, for example from M. Ivanov et al., “A simple low-SAR technique for chemical-shift selection with high-field spin-echo imaging” in Magn. Reson. Med. 64 (2010) 319-326, and Z. Nagy and N. Weiskopf, “Efficient fat suppression by slice-selection gradient reversal in twice-refocused diffusion encoding” in Magn. Reson. Med. 60 (2008) 1256-1260.
Furthermore, it is known that different spin species can have different spin-lattice relaxation times (often also called T1 relaxation time). For example, this is the case for the water signal and the fat signal. One technique that utilizes this effect of different spin-lattice relaxation times in order to suppress the fat portion is short tau inversion generation (short tau inversion recovery, STIR); see for example G. M. Bydder and I. R. Young, “MR Imaging: Clinical Use of the Inversion Recovery Sequence” in J. Comput. Assist. Tomogr. 9 (1985) 659. In the STIR technique, use is made of the fact that a previously inverted longitudinal magnetization of the spin species to be suppressed has a zero crossing at the point in time of an excitation pulse. The time period after which the excitation pulse follows the inversion pulse is often designated as an inversion time, and coincides with the spin-lattice relaxation time of the spin species to be suppressed.
Various methods for selective imaging of one or more spin species are thus known that are based either on the frequency shift or on the different T1 relaxation times. However, such techniques have diverse disadvantages and limitations. The STIR technique can require relatively long preparation times, which can increase the measurement duration. The signal-to-noise ratio of the STIR technique is typically low. The SSGR technique can have a high sensitivity with regard to spatial inhomogeneities of the basic magnetic field, for example because comparably small amplitudes of slice selection gradient pulses and/or low bandwidths of the RF pulses are selected. Moreover, an application of the SSGR technique is often limited to spin echo imaging.
It is also possible that the separation of the spin species does not take place completely, for example a residual signal of the fat component may still be visible in a water MR image. This can limit the clinical evaluation capability. A residual fat signal at the edges of an examination subject also can occur due to inhomogeneities of the basic magnetic field. This can also limit the evaluation capability of corresponding MR images.
In order to remedy such disadvantages, techniques are known that combine the STIR technique, with a partial SSGR technique often being used. “Partial” typically means that no complete suppression of a spin species to be suppressed (for instance the fat component) is achieved solely due to the SSGR portion of the combined STIR-SSGR technique. A more comprehensive suppression of the spin species to be suppressed is typically achieved only in cooperation with the STIR technique. This can make it possible to choose the amplitudes of the slice selection gradient fields to be larger so that the sensitivity to inhomogeneities of the basic magnetic field can be reduced, whereby artifacts in the MR data can be reduced in turn.
However, such combined STIR-SSGR techniques have the disadvantage that suppression of the spin species to be suppressed is often incomplete.